Alpha-olefin oligomers and their use as hydraulic fluids and synthetic lubricants (synlubes) are well known. U.S. Pat. No. 2,937,129 reports the oligomerization of C.sub.5-14 .alpha.-olefins using a dialkyl peroxide catalyst to make a synlube. U.S. Pat. No. 3,113,167 describes an .alpha.-olefin oligomer process using a titanium halide and an aluminum compound.
The preferred catalysts for making .alpha.-olefin oligomers are Friedel Crafts metal halides such as BF.sub.3, U.S. Pat. No. 3,149,178. Optimum properties are obtained starting with 1-decene although mixtures of .alpha.-olefins have been used, U.S. Pat. No. 3,330,883.
The preferred Friedel Crafts catalyst is BF.sub.3. Pure BF.sub.3 is not an effective oligomerization catalyst. A small amount of polar compound is necessary as a promoter. U.S. Pat. No. 3,382,291 describes the use of alcohol promoters such as decanol. Other reported promoters are modenite (hydrogen form), water, phosphoric acid, fatty acids (e.g. valeric acid), ketones, organic esters, ethers, polyhydric alcohols, silica gel and the like.
For use as synlubes the oligomer product is preferably a trimer or higher oligomer including mixtures thereof. Low viscosity synlubes are preferably 1-decene trimer. These have a viscosity at 100.degree. C. of about 3.4-3.7 cs (centistokes). By including a small amount (e.g. 2-10 weight percent) of tetramer the viscosity can be increased to a more desirable 3.7-4.1 cs at 100.degree. C.
It has been observed that all 1-decene trimer fractions are not the same even when made with the same catalyst. For example, use of a promoted (e.g. water, alcohol, etc.) BF.sub.3 system to oligomerize 1-decene followed by (1) topping to remove monomer and dimer and (2) distillation of the topped oligomer to remove a trimer fraction and (3) hydrogenation of the trimer fraction will give a synlube which either meets or can be adjusted to meet the 4 cs synlube viscosity specifications. However, even though the synlube exhibits the proper viscosity at -40.degree. C. (e.g. 2000-3000 cs) and 100.degree. C. (e.g. 3.6-4.2 cs), the synlube will frequently fail to meet the low temperature pour point specification. A satisfactory 4 cs synlube should have a pour point of -65.degree. C. or lower. Thus a need exists for a method for making an .alpha.-olefin synlube that will not only meet the required viscosity specifications but will also consistently exhibit a satisfactory pour point.